(a) Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having a buried layer.
(b) Description of the Prior Art
In a conventional bipolar IC (Bipolar Integrated Circuit), an epitaxial layer is formed on a semiconductor substrate with a buried region, and part of the epitaxial layer is formed as an island region electrically isolated from the remaining portion thereof. A functional element is formed in the island region. A p-n junction isolation technique, an insulator isolation technique, or a combination thereof is used to form the island region. Along with widespread application of RIE (Reactive Ion Etching) technique, a trench isolation technique is frequently employed to decrease the size of semiconductor circuits.
A conventional example using the trench isolation technique will be described with reference to FIGS. 1A to 1D.
N.sup.+ -type buried layer 12 is formed in p-type semiconductor substrate 11, and 5 .mu.m thich epitaxial layer 13 is deposited thereon. A sectional view of the resultant structure is shown in FIG. 1A. Layer 12 has a thickness of 2 .mu.m. Subsequently, 5000 .ANG. thick thermal oxide film 14 is formed on an exposed surface of layer 12 and is then etched by photolithography as a specific location to form opening 15. An oxide layer containing a diffused impurity, e.g., PSG (phosphosilicate glass) layer 16 is deposited on film 14.
The resultant structure is annealed at a high temperature for a long period of time to form so-called deep n.sup.+ -type region 17 (FIG. 1B). The impurity (e.g., phosphorus (P)) contained in layer 16 is diffused to a depth of, e.g., 2 .mu.m in layer 12. Impurity ions also leak out from layer 12 and combine with the ions from layer 16 at a depth of 2 .mu.m. Furthermore, the impurity is diffused along a direction parallel to the exposed surface for a distance corresponding to the diffusion depth in layer 12. Therefore, phosphorus ions contained in layer 16 are diffused in a 2 .mu.m wide region around opening 15 to form an N.sup.+ -type region. If opening 15 has an area of 1.times.5 .mu.m.sup.2, the combined n.sup.+ -type region has an area of 5.times.9 .mu.m.sup.2.
Subsequently, film 14 and layer 16 are removed and another thermal oxide film 18 is formed on the exposed surface of layer 13. Blocking layer 19 is formed by RIE, and annular isolation trenches 20 are formed in layer 13 (FIG. 1C). Oxide film 21 is formed on the surfaces of trenches 20, and trenches 20 ar filled with insulating layer 22 of silicon dioxide or silicon nitride to prepare the so-called island regions. Functional elements are then formed in the island regions.
The trench isolation technique was developed to decrease the size of element isolation regions. When this technique is applied to a semiconductor device with a buried layer, the buried layer is electrically connected to a surface region of the epitaxial layer by means of the impurity. As a result, lateral impurity diffusion impairs the effectiveness of the technique.
In addition, the upper half of layer 13 is converted to an n.sup.+ -type region by the impurity leaking from layer 12. Therefore, after extimating the thickness of the upper n.sup.+ -type region of layer 13, its thickness must be increased accordingly, thus increasing fabrication cost.
Another conventional example adopting the trench isolation technique is described in U.S. Pat. No. 4,140,558 issued on Mar. 2, 1978. According to this example, an isolation trench for defining an island region is formed by two steps, and an electrical connection (formation of the connection region) between the buried layer and the epitaxial surface region is made by impurity diffusion from the wall surface of the isolation trench. In this case, the connection region is formed by impurity diffusion at a relatively high temperature, i.e., 1,000.degree. to 1,050.degree. C., so that the width of the connection region is inevitably incrased. Crystal defects occur in the epitaxial layer due to oxygen leakage, and leakage of the impurity from the buried layer also occurs.